The subject matter relates to a temperature probe-a device placed on the skin of a subject to measure temperature. More particularly, the subject matter pertains to a deep tissue temperature (DTT) probe. Deep tissue temperature measurement is a non-invasive determination of the core temperature of a human body in which a probe is located over a region of deep tissue that is representative of the body core. The probe reads the temperature of that region as the core temperature.
A system for non-invasively measuring deep tissue temperature was described by Fox and Solman in 1971 (Fox R H, Solman A J. A new technique for monitoring the deep body temperature in man from the intact skin surface. J. Physiol. January 1971:212(2): pp 8-10). The system, illustrated in FIG. 1, estimates body core temperature by indirect means using a specially designed probe 10 that stops or blocks heat flow through a portion of the skin. The components of the probe 10 are contained in a housing 11. The probe 10 includes two thermistors 20 mounted on either side of a thermal resistance 22, which may be constituted of a layer of insulating material capable of supporting the thermistors 20. The probe 10 also includes a heater 24 disposed at the top of the probe 10, over the elements 20, 22, and 24. In use, the probe 10 is placed on a region of skin where deep tissue temperature is to be measured. With the bottom surface 26 of the probe resting on a person's body, in contact with the skin, the thermistors 20 measure a temperature difference, or error signal, across the thermal resistance 22. The error signal is used to drive a heater controller 30, which, in turn, operates to minimize the error signal by causing the heater 24 to provide just enough heat to equalize the temperature on both sides of the thermal resistance 22. When the temperatures sensed by the thermistors 20 are equal, there is no heat flow through the probe, and the temperature measured by the lower thermistor 20 by way of a temperature meter circuit constituted of an amplifier 36 and a temperature meter 38 is equivalent to DTT. The probe 10 essentially acts as a thermal insulator that blocks heat flow through the thermal resistor 22; DTT probes that operate in the same manner are termed “zero-heat-flux” (“ZHF”) probes. Since the heater 24 operates to guard against loss of heat along the path of measurement through the probe, it is often referred to as a “guard heater”.
Togawa improved the Fox/Solman design with a DTT probe structure that accounted for the strong multi-dimensional heat transfer of dermal blood flow through the skin. (Togawa T. Non-Invasive Deep Body Temperature Measurement. In: Rolfe P (ed) Non-Invasive Physiological Measurements. Vol. 1. 1979. Academic Press, London, pp. 261-277). The probe, illustrated in FIG. 2, encloses a ZHF sensor design 40, which blocks heat flow normal to the body, in a thick aluminum housing 42 with a disk-like construction that also reduces or eliminates radial heat flow from the center to the periphery of the probe.
Both Fox/Solman and Togawa use heat flux normal to the body (and the skin where the probe is placed) to control the operation of a heater that blocks heat flow through a thermal resistance. This results in a construction that stacks probe components, which gives the probe a substantial vertical profile. The thermal mass added by Togawa's probe design also improves the stability of the Fox/Solman design. Basic engineering for heat flux measurement would suggest that a large thermal resistance in the probe makes the measurement more accurate, but also slows the transient response rate. Since the goal is zero heat flux across the gage the more thermal resistance the better. However, additional thermal resistance adds mass and size.
Maintenance of body core temperature in a normothermic range during a perioperative cycle has been shown to reduce the incidence of surgical site infection, and so it is beneficial to monitor a patient's body core temperature before, during, and after surgery. Of course non-invasive measurement is very desirable, for both the comfort and the safety of a patient. Deep tissue temperature measurement using a probe supported on the skin provides an accurate and non-invasive means for monitoring body core temperature. However, the size and mass of the Fox/Solman and Togawa probes do not promote disposability. Consequently, they must be sterilized after each use, and stored for reuse. As a result, use of these probes to measure deep tissue temperature may raise the costs associated with DTT measurement and may increase the risk of cross contamination between patients. It is therefore useful to reduce the size and mass of a DTT probe, without sacrificing its performance, in order to promote disposability.